Nano-sized gold has been extensively investigated because physical and chemical properties of gold clusters are known to change with their size. Both bulk gold and gold nanoclusters have catalytic properties for oxidation of alcohols in alkaline medium, and for carbon monoxide. There are several reports of gold clusters containing only a few atoms being catalytically active. It has been predicted that the stability of atomic gold clusters of AuN where N=1-10 would depend on binding energy, dissociation energy, second order difference in total energy, and HOMO-LUMO energy gap. These theoretical studies show that neutral (ground state) gold clusters exhibit an odd-even oscillation of their properties due to electron-pairing effect for the second order difference in total energy and in the HOMO-LUMO energy gap. Gold clusters made of 2 and 6 atoms have the largest HOMO-LUMO gap and dissociation energy, while the second order difference in total energy is the lowest for these two, which confirms their high stability. The even numbered gold clusters are more stable than the odd numbered clusters. Moreover the Au2 and Au6 clusters have two dimensional structures. The odd-even pattern of electronic properties of gold and other coinage metals, has been confirmed in the experiment in which the UPS spectra of mass-selected metal clusters have been measured. In those experiments the oscillating pattern seizes at N>20 (for gold) and the electron affinity assumes the value corresponding to bulk metal.
It is known that catalytic performances of gold clusters depend on their preparation methods, support matrix, and their size. Both PANI containing metal precipitates and PANI modified electrodes can show electrocatalytic oxidation of primary alcohols in alkaline and acidic medium, but the catalytic effect is higher in the alkaline medium. Polyaniline is highly stable and easy to prepare, and its properties have been extensively studied. Interest in the electrocatalytic oxidation of alcohols has increased greatly due to their potential application in fuel cells. This oxidation can be catalyzed by a variety of metals, two of the most active being platinum and gold. These metals can either be used in polycrystalline form or as large, polydispersed cluster aggregates containing millions of metal atoms.
Additionally, the dramatic effect of atomic size and atomic composition of metal clusters on the electrochemical behavior of gold- or palladium-modified platinum/polyaniline electrodes has been described. The electrochemical oxidation of lower aliphatic alcohols in strongly alkaline medium has been used as the measure of electrocatalytic activity of these materials. It has been shown that the electrochemical activity correlates closely with the theoretically predicted electron affinity (HOMO-LUMO gap energy) calculated for atomic clusters in the “gas phase.” The agreement between theory and experiment has been particularly good for the homoatomic clusters of gold, and even palladium; however, somewhat weaker correlation has been obtained for hetero-atomic clusters.